One of the hitherto known methods of heating a metallic elongated material having different thickness sections is such that said metallic elongated material is placed and heated in a heating furnace. However it has been found that this conventional method should be preferably unacceptable, because during the heating in the furnace the metallic elongated material is coated with oxide film over the outer surface thereof, resulting in reduced dimensional accuracy, surface injury or damage in later hot working or irregular hardening due to reduced thermal conduction.
As an alternative method of heating a metallic elongated material induction heating has been proposed and employed to be replaced with the conventional furnace heating method. The proposed induction heating method is practiced in such a manner that the number of turns of a conductor is kept constant but voltage to be applied is variable, a feeding speed is variable or a clearance between the inductor and metallic elongated material is variable so as to achieve uniform heating over the whole length thereof. However it has been found that it is practiced only with much difficulties and after all satisfactory results fail to be obtained.
To obviate the drawback with the conventional heating methods as described above a special induction heating method was proposed and patented under Japanese Patnet Publication No. 30 570/1980.
This improved induction heating method of uniformly heating an elongated steel material over the whole length thereof consists in that when the steel material reaches a predetermined position it is detected by means of a suitable detector and then a movable heating coil moves forth and back across the heavy thickness section as if it rides thereon, while a fixed heating coil effects static heating without any movement, whereby uniform heating is ensured to a predetermined temperature. However it has been found that the above conventional induction heating method has still drawbacks, because when heating for instance a metal pipe having heavy thickness sections at both ends thereof, a temperature difference is caused between the end parts of the thin thickness section and other part of the same due to the fact that the former is heated at a temperature higher than that of the latter by way of thermal conduction from the heavy thickness sections with the result that it becomes impossible to achieve uniform heating, and moreover it has a considerable high thermal loss due to the movement of the movable heating coil. In some case where the metal pipe has an complex configuration it is difficult to effect heating.
Hence, it is an object of the present invention to provide a method and apparatus of properly induction heating a metallic elongated material having different thickness sections which is entirely free from the drawbacks with the conventional methods as described above. Specifically, the method of the invention consists in that the induction heating is continuously effected with the aid of an inductor while the metallic elongated material is displaced at a constant speed in the longitudinal direction, wherein the inductor is constructed such that the number of turns can be controlled as required and when the heavy thickness section of the metallic elongated material passes through the inductor, the effective number of turns is increased so as to ensure an increased thermal input, while when the thin thickness section passes therethrough, the effective number of turns is decreased so as to ensure a reduced thermal input, whereby the metallic elongated material is uniformly heated over the whole length thereof.
Specifically, the method of the invention is practiced with the aid of an inductor of which number of turns can be controlled as required in such a manner that when a stepped part of the metallic elongated material, that is, a boundary part intermediate a section having a heavy thickness (hereinafter referred to as heavy thickness section) and a section having a thin thickness (hereinafter referred to as thin thickness section) reaches a predetermined position it is detected by means of a suitable detector and then the detected signal is effective in increasing the number of turns of the inductor when the heavy thickness section passes therethrough, while decreasing the same when the thin thickness section does, whereby uniform heating is ensured over the whole length of the metallic elongated material by adequately controlling thermal power supply to the respective thickness sections.